PURPOSE: To reduce a life consumption rate of a piping by way of reducing thermal stress due to a temperature difference by providing a means to reduce deviation between high pressure turbine bypass pipe outlet steam temperature and high pressure turbine exhaust steam temperature at the time of starting a high pressure turbine.

CONSTITUTION: A main steam pipe 7 to supply steam from a boiler 1 to a high pressure turbine 3 and a high pressure turbine bypass pipe line 8 and a low temperature reheat steam pipe 4 to flow steam passing through these pipe line 8 and high pressure turbine 3 into each other and to supply it to a reheater 5 are provided, and steam heated by the reheater 5 is supplied to an intermediate low pressure turbine 6. Thereafter, a spary type desuperheater 9a to control pipe line outlet steam temperature to specified temperature is provided in the high pressure turbine bypass pipe line 8, and its control target setting temperature is set at outlet steam temperature 23 of the high pressure turbine 3 in correspondence with temperature rise of exhaust steam based on steam residence in the turbine until high pressure turbine exhaust steam pressure reaches steam pressure in a low temperature reheat pipe 4. Consequently, it is possible to eliminate deviation of high pressure turbine exhaust steam temperature.